This invention relates to a cooling system for an internal-combustion engine of a motor vehicle comprising a radiator and a thermostatic valve by which the temperature of the coolant can be controlled in a warm-up operation, a mixed operation and a radiator operation, the thermostatic valve containing an expansion element which can be electrically heated for reducing the coolant temperature.
In cooling systems of this type, the thermostatic valve controls the flow of the coolant between the internal-combustion engine and the radiator in the following manner. During the warm-up operation, the coolant coming from the internal-combustion engine, essentially bypasses the radiator, flowing through a short-circuit back to the internal-combustion engine. During the mixed operation, the coolant coming from the internal-combustion engine flows partially through the radiator and partially through the short circuit back to the internal-combustion engine. During the radiator operation, the coolant coming from the internal-combustion engine flows essentially through the radiator back to the internal-combustion engine. The expansion element is electrically heated to enlarge the opening cross-section toward the radiator in comparison to an opening cross-section caused by the temperature of the coolant.
A cooling system of this general type is known, for example, from German Patent Document DE 30 18 682 A1. In this cooling system, an electric heating resistor is arranged in an expansion element of a thermostatic valve. Electric energy is supplied to this electric heating resistor through a stationarily held working piston. The electric energy is supplied via a control device in order to keep the coolant temperature set by the thermostatic valve constant better than a normal thermostatic valve. Therefore, the actual coolant temperature is measured and is compared with a given upper and with a given lower temperature value. When the upper temperature value is reached, electric energy is supplied to the heating resistor so that the thermostatic valve opens up more in order to achieve an increased cooling output and therefore a lower actual coolant temperature. When the actual coolant temperature falls below the lower temperature value, the supply of electric energy to the heating resistor is interrupted so that the expansion element is cooled by the colder coolant. As a result, the valve cross-section is reduced so that the actual coolant temperature will rise. These control cycles are repeated constantly in order to keep the coolant temperature of, for example, 95.degree. C. as constant as possible.
From German Patent Document DE 37 05 232 A1, a temperature control device is known in which, instead of a conventional thermostatic valve comprising an expansion element, has a valve which can be controlled by a servomotor. In this temperature control device, the servomotor is controlled to adjust the valve as a function of a sensor which measures the coolant temperature in a pipe connected with the internal-combustion engine. In addition, the sensor has a heating device. The heating device can be switched on and off as a function of characteristic diagram quantities of the internal-combustion engine. Therefore, in the case of this temperature control device, by heating the sensor, a coolant temperature can be simulated which is higher than the real coolant temperature in order to increase the cooling of the coolant. The construction of this temperature control device requires particularly high expenditures and is therefore cost-intensive.
An object of the invention is to provide a cooling system of the initially described type that is as simple as possible so that, as a result, the operation of the internal-combustion engine can be optimized with respect to the fuel consumption and the exhaust gas values without any reduction of the power of the internal-combustion engine in the event of an increased power demand.
This and other objects are achieved by the present invention which provides an expansion element designed such that the coolant temperature is set in the mixed operation without heating the expansion element to an upper working limit temperature, and having a control unit which, as a function of sensed operating and/or environmental quantities of the internal-combustion engine, heats the expansion element, as required, in order to displace the operating mode of the cooling system in the direction of the radiator operation.
The upper working limit temperature is preferably identical to the consumption-optimal operating temperature of the internal-combustion engine and is slightly lower than the maximally permissible operating temperature of the internal-combustion engine. In certain preferred embodiments, the upper working limit temperature is above 100.degree. C., particularly at approximately 105.degree. C. The maximally permissible temperature is the highest possible temperature at which the internal-combustion engine can be operated in normal operation for an extended period of time without any disturbances. As a result, even when the electric heating of the expansion element fails, damage to the internal-combustion engine is prevented. Normally, the maximally permissible operating temperature is between 105.degree. C. and 120.degree. C.
If the expansion element is not electrically heated, an opening cross-section in the direction of the radiator occurs exclusively as a function of the coolant temperature. This opening cross-section causes a setting of the coolant temperature to the defined upper working limit temperature. By selecting a corresponding temperature-dependent material and a suitable constructive development, the expansion element is designed such that, in the case of the defined upper working limit temperature, the opening cross-section of the radiator is not yet maximal; that is, no pure radiator operation is achieved. Thus, by heating the expansion element, a further enlargement of the opening cross-section and thus a displacement in the direction of the radiator operation is possible.
In addition, the opening cross-section in the direction of the radiator and the opening cross-section in the direction of the short circuit bypassing the radiator are changed in opposite directions.
Therefore, an operating temperature of the internal-combustion engine that is as high as possible is reached in the normal operation; that is, when no increased power demand is made, such as in the full-load operation or when driving uphill. In this case, because of lower friction, the power consumption of the internal-combustion engine is less, so the fuel consumption can be lowered and the exhaust gas composition can be improved. However, in the event that the operating condition of the internal-combustion engine requires a lower coolant temperature level due to an increased power demand, the coolant temperature level may be quickly reduced. Electric energy is supplied to the heatable expansion element as a function of operating and/or environmental quantities, which further opens the thermostatic valve and as a result reduces the coolant temperature in a rapid manner. Excessive coolant or engine temperatures in the event of an increased power demand would result in a reduced volumetric efficiency and therefore in a reduced power.
In certain advantageous embodiments of the invention the control blocks the supply of electric energy to the expansion element when the sensed actual temperature of the coolant is below a predetermined desired temperature. In this case, the predetermined desired temperature is always under the defined upper working limit temperature. Thus, a control of the coolant temperature in the direction of a reduced temperature level will be carried out only when a minimum temperature has already been reached.
In certain embodiments of the invention, the control prevents the heating of the expansion element as a function of the vehicle speed. The idling can be determined when the motor vehicle is stopped, whereupon a cooling may be required because of the lack of an air stream and thus the expansion element is heated.
When a very high vehicle speed and, for example, also in addition a very large throttle valve opening angle is sensed, the conclusion is drawn that there is an increased power demand on the internal-combustion engine, whereby an increased cooling also becomes useful and thus the expansion element is heated.
In certain embodiments of the invention the control prevents the heating of the expansion element as a function of the rotational speed of the internal-combustion engine, of the throttle valve opening angle and/or the load condition of the internal-combustion engine.
For example, the control unit may compare the actual load condition and/or the actual throttle valve opening angle and/or the actual rotational speed with a predetermined threshold value and heat the expansion element when this threshold value is exceeded.
The load condition of the internal-combustion engine is determined, for example, by the rotational speed of the internal-combustion engine in conjunction with the opening angle of the throttle valve without any height correction or in connection with the air mass in the intake section with a height correction.
However, in the form of a characteristic diagram, a desired temperature of the coolant is also determinable as a function of the throttle valve angle and of the rotational speed, according to certain embodiments of the invention.
Therefore, for a high load or a high rotational speed or a large throttle valve opening angle, the required power output of the internal-combustion engine is not reduced by an excessively high operating temperature which could lead to impaired volumetric efficiency and thus to reduced power.
In certain embodiments of the invention, the control heats the expansion element when the actual temperature of the intake air or of the ambient air is above a predetermined value. Thus, in the case of high outside temperatures, for example, during slow driving, during idling when the vehicle is stopped or in a stop-and-go operation, overheating of the internal-combustion engine is prevented.
In certain embodiments of the invention the desired temperature of the coolant is taken from one or several tables, characteristic curves and/or characteristic diagrams as a function of several operating and environmental quantities. For example, for establishing a characteristic coolant temperature diagram, individual desired coolant temperatures are assigned to a plurality of operating points which are defined, for example, by values of the rotational speed of the internal-combustion engine, of the throttle valve opening angle and/or of the vehicle speed. Electric energy is supplied to the expansion element when the desired temperature taken from the characteristic diagram is below the momentary actual temperature of the coolant. Therefore, it is possible to optimize the coolant temperature at any operating point or operating condition of the internal-combustion engine.
In certain embodiments, the control unit heats the expansion element only after a predetermined operating quantity or environmental quantity hysteresis and/or after a predetermined delay time when a condition is met.
For example, in the case of a desired temperature below the actual temperature, the expansion element is heated only after a predetermined temperature hysteresis and/or a predetermined delay time.
Likewise, in certain embodiments, the control unit blocks the heating of the expansion element only after a predetermined operating quantity or environmental quantity hysteresis and/or after a predetermined delay time, when a condition is met which blocks the heating of the expansion element. For example, in the case of a desired temperature above the actual temperature, the supply of electric energy to the expansion element is blocked only after a predetermined temperature hysteresis and/or after a predetermined delay time.
By means of these embodiments of the invention, in the case of only short-term changes of the operating and/or environmental quantities, the number of control operations is reduced. This means that if a transition is to take place from the activation of the heating to a blocking of the heating and vice versa, this transition will be delayed until a longer-term change is determined.
In certain embodiments, the respectively determined desired temperature is determined essentially by a maximal temperature of the coolant which is permissible as a function of the operating and/or environmental quantities. The object of this development is to optimize the fuel consumption and the exhaust gas emissions. A highest possible operating temperature of the internal-combustion engine is adjusted which, however, as a function of the momentary load of the internal-combustion engine is determined to be only so high that damage to the internal-combustion engine or a power loss because of overheating is avoided.
In certain embodiments of the invention, an activation of the supply of the electric energy or of the heating does not necessarily result in an actual switching-on of the energy supply. An activation may also only be a switch-on option which is based on a certain condition. An actual switching-on may depend, for example, on a logic linking of several switch-on options caused by different operating and environmental quantities. Likewise, the term "blocking" may also be understood as a blocking option relative to an individual condition or as an actual switching-off.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.